Preparation of sulfones by electrolytic oxidation



Dec. 24, 1968 c. F. BENNETT ETAL 3,418,224

PREPARATION O1 SULFONES BY ELECTROLYTIC OXIDATION Filed Jan. 8, 1965 INVE NTO RS.

CLIFTON F. BENNETT DAVID W. GOHEEN BY? ATTORN United States Patent 3,418,224 PREPARATION OF SULFONES BY ELECTROLYTIC OXIDATION Clifton F. Bennett and David W. Goheen, Camas, Wash,

assignors to Crown Zellerbach Corporation, San Francisco, Calif., a corporation of Nevada Filed Jan. 8, 1965, Ser. No. 424,421

7 Claims. (Cl. 204-78) ABSTRACT OF THE DISCLOSURE This invention relates to preparation of sulfones by electrolytic oxidation of corresponding sulfoxides and organic sulfides.

In the past, electrolytic oxidation has been suggested to replace expensive chemical oxidation in the preparation of sulfones from sulfides and sulfoxides. However, a number of disadvantages have prevented this process from becoming commercially successful. Importantly, a low current efficiency (with a corresponding high current consumption) and a low yield of sulfone are characteristic. Also, the process is highly corrosive and thus destructive of the anodes used in the prior art such as platinum, steel, lead, lead dioxide, and magnetite electrodes. Of these electrodes, only prohibitively expensive platinum electrodes are able to resist corrosion to an adequate degree.

It is an object of this invention to provide a process for preparing sulfones by electrolytically oxidizing sulfoxides and organic sulfides which produces sulfone in high yield with high current efiiciency.

It is another object to provide a process for electrolytically oxidizing sulfoxides and organic sulfides to sulfones characterized by the use of an electrode which, in addition to being efiFective in producing high yields, is both inexpensive and resistant to corrosion, thereby minimizing anode investment and replacement costs and reducing both product contamination and current requirements.

These and other objects Will become apparent to those skilled in the art from the description of the invention that follows.

Electrolytic oxidation of sulfoxides and organic sulfides in acidic water solution may be remarkably enhanced in accordance with this invention by the presence in the electrolyte solution of certain catalysts which substantially increase current efficiency and yield of sulfone. Catalysts which we have found to achieve these results are vanadium, molybdenum, selenium, chromium, tungsten, and compounds thereof.

Additionally, we have discovered that a graphite or carbon anode is efiective to obtain high current efficiency and yields with a minimum of the corrosion (and thus contamination of the product) normally experienced with inexpensive electrodes heretofore considered for electrolytic oxidation of the sulfides and sulfoxides.

Although the mechanism of action of the catalysts of this invention is not yet fully understood, it is believed that the oxygen is freed at the anode to react with the sulfoxide and/or organic sulfide to form the sulfone and that, perhaps by some indirect mechanism, this reaction is promoted or catalyzed by the catalysts and the 3,418,224 Patented Dec. 24, 1968 usual side reactions, such as the formation of methane sulfonic acid, inhibited. In any event, by use of the catalysts of this invention the conversion to sulfone is increased by as much as about 37% over electrolytic oxidation under the same conditions in the absence of catalyst and yields, based upon material converted, closely approximating the theoretical may be obtained. The current efiiciency (ratio of theoretical coulom'bs to actual coulombs necessary for the oxidation) is correspondingly increased and the increase may be as high as 37%.

Turning now to the description of the process of this invention which may be batch or continuous, electrical current is passed through an electrolyte mixture by means of electrodes located therein. The electrolyte mixture comprises an electrolyte solution and an organic sulfide, sulfoxide or a combination thereof which is to be oxidized. The electrolyte solution comprises an aqueous acid solution at a desired concentration and a small amount of a catalyst. The electrical current is passed through the electrolyte mixture at a certain density, at a certain temperature and for a sufficient time to produce a desired amount of a sulfone by the electrolytic oxidation of the sulfoxides or organic sulfides. The sulfone product is separated by the conventional methods, such as recrystallization or filtration.

The amount of oxidation to the sulfone is directly proportional to the amount of current passed through the electrolytic cell. Theoretically, one faraday (96,500 coulombs; one coulomb is l ampere per second) is required to oxidize one gram-equivalent of, for instance, dimethyl sulfoxide to dimethyl sulfone. Two faradays are necessary to oxidize one mole of dimethyl sulfoxide to dimethyl sulfone.

In our process organic compounds containing a sulfoxide linkage may be employed as starting materials to be oxidized to produce the corresponding sulfone. Exemplary of such compounds are: (a) straightchain and branched chain dialkyl sulfoxides such as 'dimethy1-, methylethyl, dipropyl-, methylpropyl-, ethylpropyl-, din'butyl-', di-isobutyl-, methylhydroxyethyl, and his (hydroxyethyl) sulfoxides (b) cyclic aliphatic sulfoxides, such as trimethylene sulfoxide, tetramethylene sulfoxide, or carbon substituted derivatives thereof such as 2-nitrotetramethylene sulfoxide and bis(2-chlorocyc1ohexyl) sulfoxide (c) diaryl sulfoxides, or alkylaryl sulfoxides, such as diphenyl sulfoxide, methylphenyl sulfoxide or ringsubstituted derivatives of these compounds such as p-tolyl p-aminophenyl sulfoxides, bis(2-nitrophenyl) sulfoxide, dicresol sulfoxide, methyl-p-nitrophenyl sulfoxide, and methyl rn-chlorophenyl sulfoxide. The process is particularly suitable to use dialkyl sulfoxides to produce a corresponding sulfone. Exemplary of such sulfoxides are: dimethyl sulfoxide, methylethyl sulfoxide, diethyl sulfoxide, and carbon substituted derivatives of these compounds, such as beta hydroxy-ethyl methyl sulfoxide.

According to our method organic compounds containing a sulfide linkage may also be used to produce a corresponding sulfone. In such cases, the corresponding sulfoxide is an intermediate product which is further electrolytically oxidized into the sulfone. Exemplary of organic compounds containing a sulfide linkage are: (a) dialkyl, straight and branched chain, such as dimethyl-, diethyl-, methylethyl-, di-n-propyL, diisopropyl-, methyln-propyl-, beta-hydroxyethyl methyl sulfides which are preferred sulfides; (b) cyclic aliphatic sulfides, such as trimethylene sulfide or tetramethylene sulfide, or carbon substituted derivatives thereof; and (c) diaryl sulfides or alkylaryl sulfides, such as diphenyl-, or methylphenyl sulfides or ring-substituted derivates thereof.

To increase the efiiciency of the electrolytic oxidation, it is desirable to have high solubility of the sulfides or sulfoxides in the electrolyte mixture. In order to increase the solubility of those sulfides or sulfoxides which are not sufficiently soluble, a solvent material, such as acetic acid, methanol or water, which is miscible with the sulfide or sulfoxide and the electrolyte mixture may be added to the mixture.

Electrolytic cells for oxidation of very volatile sulfides, such as dimethyl, methylethyl and diethyl sulfides, are preferably so constructed that the losses by volatilization of the sulfides are minimized. To achieve this result, an electrolytic cell may be covered by a condenser to contain escaping vaporized sulfide and to cool and condense it to liquid which is returned to the cell.

The aqueous acidic solution, as a component of the electrolyte mixture of this invention, may contain any acid which is compatible with other components of the mixture and which will allow passage of electrical current. In general, an aqueous solution of any chemical having an acidic pH, i.e., less than 7.0 which is sufiiciently ionizable in the electrolyte mixture to allow passage of electric current when suitable voltage is applied is satisfactory for the process of this invention. Exemplary of the acid solutions are inorganic acids, such as sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, sulfurous acid. Also, organic acids such as acetic or propionic acid are suitable. However, in case of organic acids it is preferred to combine them with strong inorganic acids to increase ionization in the electrolyte mixture. Acidic salts such as sodium bisulfate or ferric chloride are also suitable. Any combination of the inorganic acids, organic acids and the acidic salts may be employed as a component of the electrolyte mixture.

The acid solution aids the passage of electrical current and the dissolving of the catalyst in the electrolyte mixture. The preferred acid solutions are those of sulfuric or hydrochloric acid in water. The concentration of the acid in the electrolyte solution may vary between about 0.1 percent and 60 percent, preferably between 0.5 percent and 2.5 percent by weight. The amount of water in the electrolyte solution may vary between 40 and 99.9 percent by weight. The amount of water required for the electrolytic oxidation to take place is one mole of water per mole of sulfoxide and 2 moles of water per mole of sulfide, and in either case at least a slight excess of water is to be used.

The catalysts used in the process of this invention are molybdenum, selenium, chromium and preferably tungsten and vanadium, and their compounds such as their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts. Exemplary of these catalysts are: sodium tungstate, sodium ortho vanadate, vanadium jpentoxide, selenium dioxide, tungstic acid, molybdic acid, chromic acid, vanadic acid, polyvanadic acid, vanadium chloride, ammonium molybdate, ammonium metavanadate, sodium tungstate diethanolamine, sodium molybdate cyclohexylamine, and sodium tungstate methylamine. The inorganic compounds of tungsten and vanadium, such as sodium tungstate and vanadium pentoxide, are preferred catalysts. One or more of the catalysts can be used.

The presence of the catalysts improves the current efiiciency and thus reduces the amount of electricity required for the oxidation by about 37%. Current efliciency is the ratio of the theoretical coulombs to the actual coulom'bs required for the oxidation of the organic sulfides or sulfoxides to the sulfones.

To employ the catalyst a small amount thereof is simply dissolved or dispersed in finely divided form in the electrolyte solution, or dissolved or dispersed in finely divided form in water or the acid solution and then is added to the electrolyte. When metallic catalysts are used, it is desirable to keep the particles of the catalyst in suspension by any conventional means such as by agitation of the electrolyte. The amount of the catalyst used may vary. However, an amount of between about 0.001 percent and about 10 percent, preferably between about 0.01 percent and 0.25 percent by weight of the organic compound being electrolytically oxidized may be sutiicient.

The process may be carried out advantageously at cell temperatures, i.e. the temperature of electrolyte mixture during the electrolysis, between above the freezing point and below the boiling point of any liquid component of the electrolyte mixture, preferably between about 15 degrees C. and about 40 degrees C.

The quantity of electrical current, preferably direct current, used is one faraday (96,500 coulombs) per one gram equivalent of the organic compound with sulfoxide linkage or two faradays per one gram equivalent of the organic compound with a sulfide linkage. Electrolysis current density, expressed in units of amperes/ sq. ft. of anode surface, is dependent on the amount of current passing through an anode of fixed surface area. The rate of oxidation is also dependent on the amount of current flowing at any given time; the number of coulombs furnished is the current in amperes per second of time. The anodic current density may be between about 10 and about 200, preferably between about 30 and about 70 amperes/ sq. ft. of anode surface exposed to the electrolyte mixture. The higher is the current density for a given area of anode the shorter is the time required for a dialkyl sulfone to be produced.

The time required for the process of this invention to produce dialkyl sulfone depends on the rate of current passage and on the current efiiciency of the electrolytic cell. The voltage across the cell varies and depends on electrical resistance of the cell and the desired electrical current employed. The current (and thus the speed of the reaction) may "be adjusted to a desired amount by applying sufiicient voltage by means of the power supply in the usual manner, such as by the use and adjustment of a rheostat.

In the methods known heretofore lead, lead dioxide, magnetite and steel have been employed as anodes. These materials decompose and corrode easily under the conditions of the process of this invention and thus appreciably contaminate the sulfone product with heavy metals or other undesirable materials. We have found that graphite and carbon anodes can be used advantageously since they allow high yield of the sulfone product and high current efiiciency without corroding or decomposing graphite or carbon and do not, therefore, contaminate the product and save the expense of purification thereof. Platinum is also a suitable material for the electrode, however, the high cost thereof renders the process of this invention very expensive.

The cathode may be of any material as long as it will permit current to flow, such as aluminum, copper, graphite, stainless steel, or platinum.

For the start up of the method of this invention in a continuous operation it is preferred to saturate the electrolyte mixture with the sulfone product which corresponds to the sulfide or sulfoxide in order to reach equilibrium condition in a short period of time.

A specific embodiment of the process of our invention may be readily understood from a description of the attached drawing which represents a diagrammatic view of a single stage apparatus for a continuous process of this invention. Referring to the drawing, an electrolytic cell 1 comprises vessel 2 in which are located electrodes 4 consisting of anode 5 and cathode 6. The electrodes are staggered in electrolyte mixture 7 to provide uniform agitation of electrolyte mixture by cascading. The electrodes are connected to a source of electric power 3. Elec trolytic cell 1 also includes electrolyte mixture 7 which is a combination of an organic sulfide or sulfoxide, an aqueous acid solution, a catalyst, and the sulfone product. Once the electric power source 3 is turned on hydrogen is liberated at cathode 6 which is the reducing electrode and oxygen is produced at anode 5 which is the oxidizing electrocle. The oxygen produced combines with the sulfide or sulfoxide in the presence of the catalyst and produces the sulfone product in electrolyte mixture 7. Electrolyte mixture 7 containing the sulfone product is discharged by conduit 8 through an air gap into outflow receiver 9. The discharge mixture 10 which is out of electrical contact with the cell is pumped into a sulfone crystallizer 11. A cooling jacket 12 around the crystallizer 11 causes the crystallization of the sulfone product which is separated on filter 14 and thus sulfone product 15 is obtained. The filtrate is discharged into make-up tank 17 whereto organic sulfide or sulfoxide 16 and other necessary components of the electrolyte mixture 7, such as catalyst, acid, and water, are added by conventional means to make up feed 18.

Organic sulfones produced by the process of this invention are useful in a variety of application, such as polymer intermediates, insecticides, pharmaceuticals, fuel additives, heat transfer agents, plasticizers, tanning agents, solvents for synthetic fibers and other polymers, and the like.

The fOllOWing examples are intended to illustrate but not to limit our invention.

Example 1 An electrolytic oxidation cell was made of a high density graphite anode in a glass beaker in which an aluminum cathode was suspended. The electrolyte mixture contained 62.0 g. (0.80 mole) of dimethyl sulfoxide, 0.4 N hydrochloric acid in 400 g. water saturated with dimethyl sulfone at 5 degrees C., and 0.013 g. of sodium tungstate powder as the catalyst dissolved therein, i.e., about 0.22 percent of the weight of the sulfoxide. The temperature of the cell was held at degrees C. The amperage was 9.7, the voltage across the cell was 3 to 6 volts, and the current density was 50 amperes per square foot of anode. The amount required of electricity was 96,500 coulombs which is the same as the theoretical amount required per gram equivalent of dimethyl sulfoxide. The electrolyte mixture was thus anodically oxidized for 4 hours and dimethyl sulfone produced was separated as follows: the solubility of the sulfone in the electrolyte was reduced by cooling the mixture to 5 degrees C. Then the sulfone which was not soluble was separated by filtering, and washed with cold ethanol (10 degrees C.), and dried. The dimethyl sulfone produced was 69.7 g. (0.74 mole). The conversion of the sulfoxide to the sulfone product was 92.5 percent. The yield was 100 percent based on the material converted and the current efiiciency was 92.5 percent.

Example 2.For comparison, without catalyst The procedure of Example 1 was repeated except that the catalyst was eliminated. The conversion to the sulfone product was 67.7 percent. The yield was 75% and the current efiiciency was 67.7 percent. There was also conversion of the sulfoxide to methane sulfonic acid and other impurities which required additional purification of the sulfone product.

This example indicates that the use of the catalyst increases current efiiciency by 36.7 percent and eliminates side reactions, thus resulting in a 33% higher yield and a 36.7% higher conversion.

Example 3 The procedure of Example 1 was repeated except that vanadium pentoxide was used as a catalyst, the amount thereof was 0.01 percent of the weight of the dimethyl sulfoxide, and before use the vanadium pentoxide was dissolved in a small amount of hydrochloric acid to make it more soluble in the form of VOC1 'Dimethyl sulfoxide employed was 88.5 g. (1.15 moles). After 6 hours of electrolytic oxidation 101.0 g. (1.07 moles) of dimethyl sulfone was produced. The conversion of the sulfoxide to the sulfone was 93.0 percent. The yield was 100 percent and the current efficiency was 93.0 percent.

Example 4.For comparison, without the catalyst The procedure of Example 3 was repeated except that the catalyst was eliminated. The conversion to the sulfone product was 67.4 percent. The yield was and the current efficiency was 67.4 percent. There was also conversion of the sulfoxide to methane sultonic acid and other impurities which required additional purification of the sulfone product.

This example indicates that the use of the catalyst increases current efiiciency by 37.4 percent and eliminating side reactions, thus resulting in a 33.3% higher yield and a 37.4 percent higher conversion.

Example 5 The procedure of Example 1 was repeated except that the catalyst used was sodium molybdate powder and the acid was 0.3 N sulfuric acid. The amount of the catalyst used was 0.005 percent of the sulfoxide weight. From 140.5 g. (1.80 moles) dimethyl sulfoxide used, 133.2 g. (1.42 moles) of dimethyl sulfone was obtained after 9 hours of electrolytic oxidation. The conversion of the sultoxide to the sulfone was 78.9 percent. The yield was percent and the current efiiciency was 78.9 percent.

Example 6 The procedure of Example 1 was repeated except that the catalyst used was selenium dioxide powder and 0.3 N sulfuric acid was employed. The amount of the catalyst used was 0.02 percent of the dimethyl sulfoxide weight. From 106.0 g. (1.36 moles) of dimethyl sulfoxide 97.5 g. (1.04 moles) of dimethyl sulfone was obtained after 10 hours of electrolytic oxidation. The conversion of sulfoxide to the sulfone was 76.5 percent. The yield was 1 percent and the current efiiciency was 76.5 percent.

Example 7 The procedure of Example 1 was repeated except that 122.2 g. (1.0 mole) of his (hydroxyethyl) sulfide was the organic compound which was oxidized, and the acid in water was saturated with his (hydroxyethyl) sulfone at 5 degrees C. The catalyst added was 0.012 g. of sodium tungstate powder, 0.01 percent of the weight of the sulfide. The temperature of the cell was held at 15 degrees C. The coulombs employed were 386,000, which was the theoretical amount needed. After 11 hours of electrolyte oxidation, 129.0 g. (0.84 mole) of his (hydroxyethyl) sulfone was produced. The conversion of the sulfide to the sulfone was 84.0 percent. The yield was 100 percent and the current efficiency was 84.0 percent.

Example 8 The procedure of Example 7 was repeated except that 138.2 g. (1.0 mole) of his (hydroxyethyl) sulfoxide was the organic compound which was oxidized. The catalyst added was 0.014 g. of sodium tungstate powder, 0.01 percent of the weight of the sulfoxide. The coulombs employed were 193,000, which was the theoretical amount needed. After 5.5 hours of electrolyte oxidation, 140.5 g. (0.91 mole) of his (hydroxyethyl) sulfone was produced. The conversion of the sulfoxide to the sulfone was 91 percent. The yield was 100 percent and the current efiiciency was 91 percent.

Example 9.-For comparison, without catalyst The procedure of Example 7 was repeated except that the catalyst was eliminated. The conversion to the sulfone product was 65.0 percent. The yield was 81 percent and the current efficiency was 65 percent. There was also a conversion of 0.19 mole of organic sulfide to impurities by side reactions.

This example indicates that the use of the catalyst increases current efficiency by 29.2 percent and eliminates side reactions, thus resulting in a 23.4 percent higher yield and a 29.2 percent higher conversion.

From the foregoing description and examples it is apparent that the presence of the catalysts employed in the process of the present invention produces up to about 37 percent more dialkyl sulfone in a given time and a much greater current efficiency than that produced by the same or similar processes in the absence of the catalyst.

We claim:

1. A process of electrolytically producing a sulfone which comprises subjecting an organic compound selected from the group consisting of organic sulfides, sulfoxides I and a combination thereof to electrolysis in an electrolyte mixture comprising an aqueous acid solution and a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts.

2. A process of electrolytically producing a sulfone which comprises subjecting an organic compound selected from the group consisting of organic sulfides, sulfoxides and a combination thereof to electrolysis in an electrolyte mixture comprising an aqueous acid solution and a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts in the amount of between about 0.001 and about 10 percent by weight of the organic compound.

3. A process of electrolytically producing a sulfone which comprises subjecting an organic compound selected from the group consisting of organic sulfides, sulfoxides and a combination thereof to electrolysis in an electrolyte mixture comprising an aqueous acid solution and a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts; and keeping an electrolysis current density of between about 10 and about 200 amperes per square foot of the anode surface exposed to the electrolyte mixture.

4. A process of electrolytically producing a sulfone which comprises subjecting an organic compound selected from the group consisting of organic sulfides, sulfoxides and a combination thereof to electrolysis in an electrolyte mixture comprising an aqueous acid solution in the presence of a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts; and in the presence of an anode composed of a material selected from the group consisting of graphite and carbon.

.5. A process of electrolytically producing a dimethyl sulfone which comprises subjecting dimethyl sulfoxide to electrolysis in an electrolyte mixture comprising an aqueous acid solution in the presence of a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts; and in the presence of an anode composed of a material selected from the group consisting of graphite and carbon.

6. A process of electrolytically producing a sulfone which comprises subjecting an organic compound selected from the group consisting of organic sulfides, sulfoxides and a combination thereof to electrolysis in an electrolyte mixture comprising an aqueous acid solution in the presence of a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium and amine salts; and in the presence of an anode composed of a material selected from the group consisting of graphite and carbon while maintaining a cell temperature of above the freezing point and below the boiling point of any liquid component of the electrolyte mixture.

7. A process of electrically producing a dimethyl sulfone which comprises subjecting dimethyl sulfoxide to electrolysis in an electrolyte mixture containing an aqueous acid solution in the presence of a catalyst selected from the group consisting of tungsten, vanadium, molybdenum, selenium, chromium, and their oxides, acids, alkali metal salts, alkaline earth metal salts, ammonium salts and amine salts; and in the presence of an anode composed of a material selected from the group consisting of graphite and carbon; keeping an electrolysis current density at the anode of between about 10 and about 200 amperes per square foot of the anode surface exposed to the electrolyte mixture; and maintaining a cell temperature of above the freezing point and below the boiling point of any liquid component of the electrolyte mixture.

References Cited UNITED STATES PATENTS 2,521,147 9/1950 Brown 204-79 3,200,054 8/1965 Pursley 20479 OTHER REFERENCES Technique of Organic Chemistry, vol. II, Catalytic, Photochemical, Electrolytic Reactions, 2nd edition (1956), pp, 404, 442 and 460.

HOWARD S. WILLIAMS, Primary Examiner. 

